When he was just a kid, he needed to learn how survive alone, and many times suffered in bad hands, until he became the cruel Flame manipulator - any flame, from the normal until the dark ones from Makai -

Hiei always lived alone, by himself and became a cold person, lonely and of few words. And to complete, his personality is strong enough to use cruel words with any people - a friend or an enemy - who is of his side.

When he grew up a little more, Hiei discovered that his twin sister had disappeared and, to find her, he got his Jagan, the third eye. This operation was very painful, but thanks to it, Hiei's power grew up a lot. So, the youkai started to search for his disappeared sister, and in one of his travels to the Ningenkai, he knew Kurama, who was 14 years old.

After this meeting, Hiei disappeared for a long time, and reappeared to Kurama when he was fifteen years old to convince the youko to help him and Goki in the theft of the Spiritual Treasures...in this time, he knew Yusuke and after that, Kuwabara.

Slowly, he began to help Yusuke's group in defending Ningenkai from the other worlds, always saying something like: This demons steeled some treasures mine in the past, that's why I want to kill them..." or something like this.

And one day he found his sister, Yukina, but he didn't tell her he's her brother...In the last episodes of the series, there are a scene where I think Yukina knows the truth...because she gives to him her tear-gem, and she talks to Hiei as if she was talking to her brother...IMO, she's much smarter than she looks...^^
 

Hiei's techniques are based in Jia Ou Ensatsu Ken, a group of attacks that use the fire like power font. The most powerful of the secrets is the Jia Ou Ensatsu Kokuryuha, the Black Dragon summoned from the dark world, who uses Hiei's Youki like way to come to the Ningenkai. This attack is terrible, and the power is unlimited. Hiei uses this technique in his first fight at the Ankoku Bujutsukai against Zeru. Them, he uses it one more time against Bui, from Toguro's Group. This time, his attack is much better and becomes more and more perfect while Hiei learns how to control it...

His other techniques are Jia Ou Ensatsu Ken (the same name of the group of techniques, but the last ken is for sword). It is a Dark Fire Sword, that combines the dark fire with the Youki. Hiei doesn't like this attack, because it's too similar with the Rei Ken, Kuwabara's attack. Ensatsu Rengoku Shou is one or more fire punches. This attack uses only the fire of the Ningenkai, but it's very destructive too...(we can't compare it to the Black Dragon, of course!).